1. Field of the Invention
The present invention relates to a color thermal printing method, particularly suitable for a direct color thermal printing method.
2. Description of the Background Art
Color thermal printing methods include a thermal transfer printing method using a color ink ribbon or a color ink sheet and a direct color thermal printing method using a color thermosensitive recording sheet.
In the thermal transfer printing method, a color ink ribbon or color ink sheet is placed on a recording sheet (image reception sheet), and the back of the color ink ribbon or color ink sheet is heated to transfer color ink onto the recording sheet. The thermal transfer printing method includes two types, one type sublimating color ink and transferring it onto a recording sheet, the other type melting color ink and transferring it onto a recording sheet.
A color ink ribbon is used for a serial printer which performs three color line sequential printing, and has a yellow ink area, a magenta ink area, and a cyan ink area formed in a predetermined order over one line length. A color ink sheet is used for a line printer which performs three color frame sequential printing. Three color sheets of yellow, magenta, and cyan are used for printing a full-color image. In addition to three primary colors, there are a four color ink ribbon and four color ink sheets containing black color.
The sublimation transfer type is suitable for printing a full-color image because the density of an ink dot of one pixel can be changed with heat energy. The thermal wax transfer type is suitable for printing characters and lines because the density of an ink dot cannot be changed. When printing a half tone color image by a thermal wax transfer type printing method, an area gradation method is used. With this area gradation method, one pixel is divided into a plurality of subsidiary lines, and a subsidiary ink dot is selectively transferred onto each subsidiary line. One ink dot of one pixel is constituted by these subsidiary ink dots. Each ink dot changes its area in accordance with a gradation level. Of the plurality of subsidiary lines, the first subsidiary line faces a heating element first. In transferring a subsidiary dot onto this first subsidiary line, a wide drive pulse is used in order to raise the temperature of the heating element to an ink transfer temperature. In transferring subsidiary ink dots onto the second and following subsidiary lines, narrow drive pulses are used which are sufficient for maintaining the heating element at the ink transfer temperature.
With the direct color thermal printing method, a color thermosensitive recording sheet is directly heated to develop colors. The density of an ink dot of one pixel changes with heat energy. As described, for example, in U.S. Pat. No. 4,734,704 (corresponding to JP-A 61-213169), a color thermosensitive recording sheet has a magenta thermosensitive coloring layer, a cyan thermosensitive coloring layer, and a yellow thermosensitive coloring layer, respectively formed on a base in this order. In the color thermosensitive recording sheet, the lower the thermosensitive coloring layer, the lower the heat sensitivity. Each thermosensitive coloring layer is optically fixed by electromagnetic rays having a wavelength specific thereto. A coloring heat energy is required to record an ink dot on one pixel. This coloring heat energy is a sum of a heat energy of a level immediately before coloring (hereinafter called a bias heat energy which is changed with color) and a heat energy for coloring at a desired density (hereinafter called an image heat energy).
For example, in printing a full-color image on a color thermosensitive recording sheet, a thermal head is used which has a plurality of heating elements disposed in line in the main scan direction. For example, a color thermosensitive recording sheet is wound about a platen drum, and is moved in the subsidiary scan direction by rotating the platen drum. While the recording sheet is moved, a thermal head heats it and prints a yellow image on the yellow thermosensitive coloring layer. After this printing, the yellow thermosensitive coloring layer is optically fixed by applying light having a wavelength which decomposes only a diazonium salt compound of the yellow thermosensitive coloring layer. Next, a coloring energy higher than that of the yellow thermosensitive coloring layer is applied to the magenta thermosensitive coloring layer so as to print a magenta image on the magenta thermosensitive coloring layer. After this printing, the magenta thermosensitive coloring layer is optically fixed by applying light having a wavelength which decomposes only a diazonium salt compound of the magenta thermosensitive coloring layer. Lastly, a largest coloring heat energy is applied so as to print a cyan image on the cyan thermosensitive coloring layer.
With a conventional direct color thermal printing method, the three color thermosensitive coloring layers are sequentially caused to record while starting at the same print start position on the color thermosensitive recording sheet. As a result, the print length in the subsidiary direction changes with color, and the color registration shift amount becomes larger towards the trailing edge of the image frame of the recording sheet. This color registration shift causes a change in the tone of a color image and an unsharp image.
It is hypothesized that a change in the print length in the subsidiary direction results from an expansion/contraction of a color thermosensitive recording sheet generated by a printing heat. For example, if polyethylene terephthalate (PET) is used as the material of the base of a color thermosensitive recording sheet, the base is expanded temporarily by a printing heat, and recovers the original condition after printing. As shown in FIG. 16, the printed length AY of a yellow image printed with a smallest coloring heat energy is shortest because a contraction during printing is smallest. The printed length AC of a cyan image printed with a largest coloring heat energy is longest because a contraction during printing is largest. A difference between printed lengths is one to two lines or one to two pixels on an A6 size color thermosensitive recording sheet.
In both the cases of the thermal transfer printing method and direct color thermal printing method, a thermal head is cold before printing so that its temperature will not rise to a predetermined value just at printing time. As a result, the density at the print start area is low which generates a so-called shading, i.e., difference in density between positions on the recording sheet.
A rubber roller is used as a platen drum in a general thermal printer. Rotation force of a pulse motor is transmitted via a belt to the platen drum. Immediately after the start of rotation, the platen drum is deformed because of the elastic nature of rubber and cannot rotate without being in the deformed state. The belt has some play. Because of the deformation of the platen drum and the play of the belt, feeding of a recording sheet becomes unstable immediately after the start of rotation, or in some cases a recording sheet is not fed at all. With the thermal transfer printing method, if a recording sheet is stopped over several lines and a thermal head is driven, the heating elements heat an ink ribbon or ink sheet a plurality of times at an identical position. In such a case, the base of the ink ribbon or ink sheet may be dissolved and attached to the heating elements, resulting in a poor heat conduction property of the heating elements and in some cases in a disability of printing. With the direct color thermal printing method, if heat energies for a plurality of pixels are applied to the same area, the underlying thermosensitive coloring layer may develop color, resulting in mixed colors. In both the printing methods, a color registration shift is generated if feeding a recording sheet is unstable.
In order to solve the above problems, various methods have been proposed. For example, Japanese Patent Laid-open Publication No. 61-94453 has proposed a method of starting printing at a timing delayed from the start of rotation of a platen drum, without starting printing at the same time when the platen drum is rotated. More specifically, a short preliminary running area is formed upstream of the print area. In the preliminary running area, the platen drum is rotated while a thermal head is pressed against a recording sheet. Under this condition, the platen drum is deformed by a maximum friction force of the thermal head pressed against the recording sheet, and the mechanical play of the belt and the like is absorbed. It becomes possible therefore to feed a recording sheet stably in the print area.
A recording sheet is generally wound about a platen drum which is slightly swelled on the clamper side. This swell is gradually diminished as the platen drum rotates further. This swell results in unstable feeding of a recording sheet, and the state of a swell is not always constant. Because of these reasons, a color registration shift is generated in three color frame sequential printing. In view of this, another method has been proposed, for example, in Japanese Patent Laid-open Publication No. 62-127255 wherein a recording sheet is fed a short preliminary distance while being pressed against the platen drum by a thermal head.
However, as described above, even if a recording sheet is fed short preliminary running distance while being pressed by the thermal head, a color registration shift cannot be removed completely.
A friction coefficient between a recording sheet and a thermal head changes with a heat energy generated by the thermal head. As shown in FIG.15, as the temperature of a thermal head rises, a friction coefficient tends to be lowered. Therefore, the friction coefficient lowers instantly when the thermal head enters the print area so that feeding of a recording sheet changes and a pixel position is displaced. The direct color thermal printing method uses a different heat energy for each recording color. Therefore, a large change in the friction coefficient occurs between the preliminary running area and print area, and hence a large color registration shift occurs.